'Islamic State' extremists murder second US reporter

In the latest footage, the 31-year-old reporter Steven Sotloff calmly addresses the camera to say he is a victim of President Barack Obama's decision to press on with air strikes in Iraq against the jihadists.

Obama later Tuesday authorized about 350 more American troops to beef up security at US diplomatic facilities and protect personnel in the Iraqi capital Baghdad.

At the end of the five-minute video recording, discovered online by the SITE terrorism monitoring group and seen by AFP, the militant threatens another captive, identified as British citizen David Cawthorne Haines.

"I'm back, Obama, and I'm back because of your arrogant foreign policy towards the Islamic State," the black-clad jihadist says, wielding a combat knife and speaking in a London accent.

This was a reference to a video issued last month in which US journalist James Foley was murdered, again by a suspected British foreign fighter, and in almost identical fashion.

'Despicable act'

The fighter condemned recent US air strikes on the area around the Mosul Dam in Iraq, dating the footage after the 40-year-old Foley's killing.

"So just as your missiles continue to strike the necks of our people, our knife will continue to strike the necks of your people," he declared, before reaching round to cut his captive's throat.

Sotloff's hair and beard were longer than in previous IS footage, in which he was threatened with death in retaliation for US strikes against the Sunni extremist group, which has declared an Islamic "caliphate" in regions under its control in Iraq and Syria.

In a warning to Britain, the killer declared: "We take this opportunity to warn those governments that enter this evil alliance of America against the Islamic State to back off and leave our people alone."

The Sotloff family, who live in Miami, issued a statement through a spokesman, Barak Barfi, that implicitly confirmed the video as authentic.

"The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time," it said.

After Foley's death, Sotloff's mother Shirley had addressed a video message to IS leader Abu Bakr al-Baghdadi pleading for her son's life, and insisting he had no influence on US policy.

A US spokeswoman said Washington was trying to authenticate the "sickening" footage, and British Prime Minister David Cameron said it depicted an "absolutely disgusting, despicable act."